System and method for calibration of volatile organic compound detecting instruments

ABSTRACT

Embodiments relate generally to systems and methods for calibrating a gas detector. Some embodiments may include a gas detector comprising one or more sensing elements operable to detect volatile organic compounds (VOC), and a probe operable to provide air flow into the gas detector, wherein the probe comprises: a housing operable to attach to a main body of the gas detector and operable to hold a calibration tube during calibration of the gas detector; a cap operable to attach to the housing and operable to fit over and cover the calibration tube; and one or more inlets, wherein when a calibration tube is inserted into the probe, the air flows through the calibration tube. In some embodiments of the gas detector, the calibration tube inserted into the probe comprises activated carbon fiber (ACF) filter material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.15/184,388, filed Jun. 16, 2016 and entitled “System and Method forCalibration of Volatile Organic Compound Detecting Instruments” byChunyu Shao et al. which is incorporated herein by reference in itsentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

Gas detectors may be carried by workers and/or located throughout a workplace and may detect gases in the environment. Gas detectors may beoperable to alert a user and/or supervisor when a harmful gas or levelof gas is detected. To ensure that gas detectors are functioningproperly, the detectors may be periodically calibrated (or zeroed). Somecalibration methods may involve providing purified or filtered air tothe gas detector and calibrating the resulting readings to a zeroreference point.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, referenceis now made to the following brief description, taken in connection withthe accompanying drawings and detailed description, wherein likereference numerals represent like parts.

FIG. 1 illustrates a calibration tube according to an embodiment of thedisclosure;

FIGS. 2A-2B illustrate a calibration tube and packaging according to anembodiment of the disclosure;

FIG. 3 illustrates a method according to an embodiment of thedisclosure;

FIG. 4 illustrates a gas detector according to an embodiment of thedisclosure;

FIG. 5 illustrates a probe according to an embodiment of the disclosure;and

FIG. 6 illustrates the probe of FIG. 5 attached to a gas detectoraccording to an embodiment of the disclosure.

DETAILED DESCRIPTION

It should be understood at the outset that although illustrativeimplementations of one or more embodiments are illustrated below, thedisclosed systems and methods may be implemented using any number oftechniques, whether currently known or not yet in existence. Thedisclosure should in no way be limited to the illustrativeimplementations, drawings, and techniques illustrated below, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

The following brief definition of terms shall apply throughout theapplication:

The term “comprising” means including but not limited to, and should beinterpreted in the manner it is typically used in the patent context;

The phrases “in one embodiment,” “according to one embodiment,” and thelike generally mean that the particular feature, structure, orcharacteristic following the phrase may be included in at least oneembodiment of the present invention, and may be included in more thanone embodiment of the present invention (importantly, such phrases donot necessarily refer to the same embodiment);

If the specification describes something as “exemplary” or an “example,”it should be understood that refers to a non-exclusive example;

The terms “about” or “approximately” or the like, when used with anumber, may mean that specific number, or alternatively, a range inproximity to the specific number, as understood by persons of skill inthe art field; and

If the specification states a component or feature “may,” “can,”“could,” “should,” “would,” “preferably,” “possibly,” “typically,”“optionally,” “for example,” “often,” or “might” (or other suchlanguage) be included or have a characteristic, that particularcomponent or feature is not required to be included or to have thecharacteristic. Such component or feature may be optionally included insome embodiments, or it may be excluded.

Embodiments of the disclosure include systems and methods forcalibrating a gas detector. Some gas detectors may be operable to detectvolatile organic compounds (VOCs). Indoor and outdoor VOCs endangerhuman health, and increases of VOC concentration in productionenvironments may decrease product quality and yield. To ensure safetyand efficiency, it is important to quickly and accurately detect traceamounts of total VOC (TVOC) concentration levels (such as in parts permillion (ppm) and/or parts per billion (ppb)). A photoionizationdetector (PID) (which may also be referred to more generally as a “gasdetector”) may be typically used for TVOC detection, and may becharacterized by high sensitivity (achieving ppb level resolution),quick response time (generally a few seconds), and a wide range ofdetectable substances (the majority of VOCs can be detected by PID).However, there are some issues in current PID rapid detection. PIDs needto be regularly zero-calibrated because of the tendency for zero driftin the sensor. Alternatives to zero calibration include using cylinderclean air and/or using a filter, but both of these solutions havedisadvantages. When using cylinder clean air, it may be troublesome toprepare and preserve very pure air, and it may be inconvenient and highcost.

When using a filter during calibration, activated carbon may be used asadsorbent to make the VOC zeroing tube. The filter purifies ambient airfor zero calibration and provides a zero standard, and may be low costand easy to use. However, typical calibration tubes having activatedcarbon filters not only remove VOC from the ambient air, but also removewater vapor. This causes the humidity of the calibration gas to bedifferent from the humidity of the sample gas, which may affect theaccuracy of the readings of the PID. For some rapid detection means,such as PID, water vapor will affect the response. Therefore, thehumidity difference between filtered calibration gas and sample gas willlead to reading deviation.

Disclosed embodiments may comprise systems and methods for humidifyingthe filter material in a calibration tube for use with a PID. Thecalibration filter material may be pretreated to maintain the humiditylevels of the ambient air. The absorbent material for the calibrationtube may comprise a porous material with a large specific surface area,and the material may not be strongly polar or absolutely non-polar(where strongly polar materials may absorb too much water from thefiltered gas, and non-polar materials cannot absorb polar VOC).Additionally, the material may be intrinsically stable, operable toquickly absorb VOC gas and not release the absorbed VOCs.

In some embodiments of the calibration tube, the filter material maycomprise activated carbon fibers (ACF), which may have high adsorbingefficiency and quick humidity equilibrium speed due to the unique meshfabric structure and micro-pores of the material, compared with granularactivated carbon. The ACF is designed to quickly generate relativelypure gas by a special pretreatment method as described herein.Embodiments of the calibration tube may comprise activated carbon fiberas adsorbent material, where the ACF may be pretreated, therebyhumidifying the adsorbent.

In some embodiments, the humidity of the gas entering the PID may bemaintained by the calibration tube at approximately 50% relativehumidity (RH). In other embodiments, the humidity may be maintained bythe calibration tube at any point between approximately 10-90% RHdepending on the use and application of the calibration tube.

Referring now to FIG. 1, an exemplary embodiment of a calibration tube100 is shown. The calibration tube 100 may comprise a glass casing 102,wherein a portion of the glass casing 102 may be broken before the tube100 is used. In an embodiment, the calibration tube 100 may comprise afilter material 104, wherein the calibration tube 100 may be operable tofilter ambient air to generate a zero standard for calibrating a gasdetector. In some embodiments, the filter material 104 may comprise ACF(as described above). In some embodiments of the calibration tube 100,the ACF filter material 104 may be pretreated to enable the filtermaterial to maintain a humidity level greater than zero, and near theambient humidity level. The calibration tube 100 may comprise a firstend 106 and a second end 108. The ends 106 and 108 of the calibrationtube may be broken prior to use of the tube, to allow air to flow intothe calibration tube 100 and then into the gas detector (or PID) tocalibrate the gas detector.

FIGS. 2A-2B illustrate a method of breaking a portion of the calibrationtube 100 (such as one or more of the ends 106 and 108). The calibrationtube 100 may be packaged with other tubes in a package box 200, wherethe tubes may be delivered to users in these boxes 200. In anembodiment, the box 200 may comprise a “tube tip breaker” 202 located onthe exterior of the box 200. The tube tip breaker 202 may comprise ametal ring, hole, or other similar material operable to break the glassmaterial of the tube 100. The tube 100 may be inserted by a user intothe tube tip breaker 202 and twisted, turned, and/or bent by the user(such as in the direction indicated by arrow 210). The first end 106 ofthe tube 100 that is inserted into the tube tip breaker 202 may bebroken, allowing air flow into and/or out of the calibration tube 100.FIG. 2B illustrates a closer view of the tube tip breaker 202. In someembodiments, both ends 106 and 108 of the tube 100 may be broken usingthis method before the calibration tube is used with a gas detector.

Referring to FIG. 3, a method 300 for assembling the calibration tube(described above) is shown. At step 302, a pretreatment saturatedsolution of sodium bromide (NaBr) may be prepared. In some embodiments,the NaBr solution may be made up as a slushy mixture with distilledwater and chemical pure salt. At step 304, the NaBr solution may beenclosed in a sealed clean glass chamber, keeping the surface area ofthe solution as large as possible. At step 306, ACF may be desorbed inan oven at approximately 160° C. and kept heated at that temperature forapproximately 1.5 hrs. At step 308, the heated ACF may be moved into theglass chamber containing the NaBr solution, wherein the glass chambermay be sealed. The ACF and NaBr solution mixture may be allowed toequilibrate humidity for approximately 40 to 48 hours. In someembodiments, ACF and NaBr solution mixture may be allowed to equilibratefor at least 24 hours. Then, at step 310, the ACF that has beenpretreated with the NaBr solution may be placed in a glass calibrationtube, which at step 312, may be immediately sealed.

To complete calibration of a gas detector (such as a PID) using acalibration tube, the two ends of the glass calibration tube may bebroken (as described above), and the calibration tube may be insertedinto the air flow entering the gas detector. In some embodiments, thegas detector may comprise a probe that provides air flow to the sensorelements of the gas detector. Typically, an adapter may be used toattach a calibration tube to the probe, such that air entering the probefirst passes through the calibration tube. However, the process ofattaching the tube may be complicated, requiring multiple separateelements, and one end of the broken glass tube is exposed, creating thepotential for cutting of a user's hands by accident while completing thecalibration.

Embodiments of the disclosure include a probe structure that allows foran efficient and safer calibration process using a glass calibrationtube.

FIG. 4 illustrates an exemplary embodiment of a gas detector 600 wherethe gas detector 600 comprises a probe section 400. During normal use,the gas that is sensed by the gas detector 600 may enter the gasdetector 600 via the probe 400. In the embodiment shown, the probe 400may comprise an elongated, thin component extending from the main bodyof the gas detector 600. Typically, to calibrate the gas detector 600, acalibration tube (such as the calibration tube 100 described above) maybe coupled to the probe 400 in some way, such that the gas sensed by thegas detector 600 is filtered by the calibration tube and/or is receivedonly from the calibration tube.

FIG. 5 illustrates an exemplary embodiment of a probe 400, wherein theprobe 400 is operable to attach to a gas detector (such as the gasdetector 600 described above). The probe 400 may comprise a housing 402and cap 404, wherein the housing 402 may be operable to attach to a gasdetector, and the cap 404 may be operable to thread onto and off of thehousing 402 via threads 403 and 405. In some embodiments, the cap 404and housing 402 may attach via another attachment means, such as a clip,adhesive, a snap mechanism, etc. The attachment between the housing 402and cap 404 may allow for easy and efficient removal and reattachment ofthe cap 404 to/from the housing 402. In some embodiments, the housing402 and/or cap 404 may comprise grooves 414 and 416 (or another grippingfeature) providing a secure gripping surface for the user.

The probe 400 may comprise an inlet 411 near the top of the probe 400.In some embodiments, the probe 400 may comprise secondary inlets 412 and410 located on at least one of the housing 402 and/or the cap 404,operable to let gas into the probe 400, particularly if something isblocking the top inlet 411 of the probe 400. In some embodiments, thehousing 402 may comprise inlets 410, and the cap 404 may comprise inlets412. In some embodiments, only the cap 404 may comprise inlets 412. Insome embodiments of the probe 400, the housing 402 may attach to the gasdetector, and the probe 400 may comprise an O-ring 408 and/or supportblock 406 located at the attachment with the gas detector. In someembodiments, the support block 406 may comprise a PTFE material. In someembodiments, the O-ring 408 may be operable to seal against the end of acalibration tube that is inserted into the housing 402.

FIG. 6 illustrates another view of the probe 400 attached to a gasdetector 600 via the connection point 602. In the embodiment shown inFIG. 6, a calibration tube 500 (similar to calibration tube 100 shown inFIG. 1) may be inserted directly into the housing 402 of the probe 400.In some embodiments, the ends (or tips) 508 of the calibration tube 500may be broken before the calibration tube 500 is inserted into the probe400, such that the air flow into the gas detector 600 may pass throughthe calibration tube 500. To secure the probe 400, and also to protect auser from the broken glass ends 508 of the tube, the cap 404 may beattached over the calibration tube 500 to the housing 402, covering thetube 500. Then, calibration of the gas detector 600 may be completed,and the calibration tube 500 may be held securely and safely within theprobe 400. In some embodiments, the calibration tube 500 may comprise afilter material 504 (similar to the filter material 104 describedabove).

When calibration is completed, the calibration tube 500 may be removedfrom the probe, the cap 404 may be reattached to the housing 402, andnormal detection by the gas detector 600 may proceed. In an alternativeembodiment, the normal detection by the gas detector 600 may proceedwith only the housing 402 attached to the gas detector 600. In anotherembodiment, the probe 400 may comprise a “calibration” cap used duringcalibration and a “detection” cap used during normal detection thatprovide different functionality for the probe 400 depending on the use.

Some embodiments of the disclosure may comprise a gas detectorcomprising one or more sensing elements operable to detect volatileorganic compounds (VOC) and a probe operable to provide air flow intothe gas detector, wherein the probe comprises: a housing operable toattach to a main body of the gas detector and operable to hold acalibration tube during calibration of the gas detector; a cap operableto attach to the housing and operable to fit over and cover thecalibration tube; and one or more inlets, wherein when a calibrationtube is inserted into the probe, the air flows through the calibrationtube.

In an embodiment of the gas detector, the calibration tube inserted intothe probe comprises activated carbon fiber (ACF) filter material. In anembodiment of the gas detector, the calibration tube inserted into theprobe comprises ACF filter material that has been pretreated with asodium bromide (NaBr) solution. In an embodiment of the gas detector,the calibration tube inserted into the probe comprises broken glassends, allowing air flow into and out of the calibration tube. In anembodiment of the gas detector, the probe further comprises threadsbetween the housing and the cap. In an embodiment of the gas detector,the probe further comprises gripping elements on the exterior of atleast one of the housing and cap. In an embodiment of the gas detector,the probe further comprises an O-ring operable to seal with one end ofthe calibration tube when it is inserted into the housing. In anembodiment of the gas detector, the probe further comprises air inletson the housing. In an embodiment of the gas detector, the probe furthercomprises air inlets on the cap. In an embodiment of the gas detector,the probe further comprises a support block operable to seal the probewith the gas detector.

Some embodiments of the disclosure may comprise a method for completinga calibration process for a gas detector comprising providing a gasdetector comprising a probe, wherein the probe comprises a housingoperable to attach to a main body of the gas detector and operable tohold a calibration tube during calibration of the gas detector; a capoperable to attach to the housing and operable to fit over and cover thecalibration tube; and one or more inlets, wherein when the calibrationtube is inserted into the probe, the air flows through the calibrationtube; breaking a portion of the calibration tube to allow air flowthrough the calibration tube; inserting the calibration tube into thehousing of the probe; attaching the cap of the probe onto the housing,wherein the cap covers the calibration tube; completing calibration ofthe gas detector while the calibration tube is inserted into the probe;removing the calibration tube from the probe; and receiving gas readingsby the gas detector.

In an embodiment of the method, the method may further compriseproviding an activated carbon fiber (ACF) filter material; pretreatingthe ACF filter material with a sodium bromide (NaBr) solution; placingthe ACF filter material into the calibration tube; and sealing thecalibration tube. In an embodiment of the method, pretreating the ACFfilter material comprises heating the ACF to 160° C. for approximately1.5 hours; enclosing the ACF filter material in a glass chamber with aNaBr solution; and allowing the ACF filter material and NaBr solution toequilibrate for at least 24 hours. In an embodiment of the method, thegas detector is operable to detect volatile organic compounds (VOC). Inan embodiment of the method, breaking a portion of the calibration tubecomprises breaking a first tip on one end of the tube; and breaking asecond tip on the other end of the tube. In an embodiment of the method,attaching the cap of the calibration tube onto the housing comprisesthreading the cap onto the housing.

Some embodiments of the disclosure may comprise a method formanufacturing a calibration tube for use with a gas detector comprisingproviding an activated carbon fiber (ACF) filter material; pretreatingthe ACF filter material with a sodium bromide (NaBr) solution; placingthe ACF filter material into the calibration tube; and sealing thecalibration tube.

In an embodiment of the method, pretreating the ACF filter materialcomprises heating the ACF to 160° C. for approximately 1.5 hours,enclosing the ACF filter material in a glass chamber with a NaBrsolution, and allowing the ACF filter material and NaBr solution toequilibrate for at least 24 hours. In an embodiment of the method, themethod may further comprise breaking a portion of the calibration tubeto allow air flow through the calibration tube; inserting thecalibration tube into a first portion of a probe attached to the gasdetector; attaching a second portion of the probe onto the firstportion, wherein the second portion covers the calibration tube; andcompleting calibration of the gas detector while the calibration tube isinserted into the probe. In an embodiment of the method, the method mayfurther comprise employing the gas detector to detect volatile organiccompounds (VOC).

While various embodiments in accordance with the principles disclosedherein have been shown and described above, modifications thereof may bemade by one skilled in the art without departing from the spirit and theteachings of the disclosure. The embodiments described herein arerepresentative only and are not intended to be limiting. Manyvariations, combinations, and modifications are possible and are withinthe scope of the disclosure. Alternative embodiments that result fromcombining, integrating, and/or omitting features of the embodiment(s)are also within the scope of the disclosure. Accordingly, the scope ofprotection is not limited by the description set out above, but isdefined by the claims which follow, that scope including all equivalentsof the subject matter of the claims. Each and every claim isincorporated as further disclosure into the specification, and theclaims are embodiment(s) of the present invention(s). Furthermore, anyadvantages and features described above may relate to specificembodiments, but shall not limit the application of such issued claimsto processes and structures accomplishing any or all of the aboveadvantages or having any or all of the above features.

Additionally, the section headings used herein are provided forconsistency with the suggestions under 37 C.F.R. 1.77 or to otherwiseprovide organizational cues. These headings shall not limit orcharacterize the invention(s) set out in any claims that may issue fromthis disclosure. Specifically and by way of example, although theheadings might refer to a “Field,” the claims should not be limited bythe language chosen under this heading to describe the so-called field.Further, a description of a technology in the “Background” is not to beconstrued as an admission that certain technology is prior art to anyinvention(s) in this disclosure. Neither is the “Summary” to beconsidered as a limiting characterization of the invention(s) set forthin issued claims. Furthermore, any reference in this disclosure to“invention” in the singular should not be used to argue that there isonly a single point of novelty in this disclosure. Multiple inventionsmay be set forth according to the limitations of the multiple claimsissuing from this disclosure, and such claims accordingly define theinvention(s), and their equivalents, that are protected thereby. In allinstances, the scope of the claims shall be considered on their ownmerits in light of this disclosure, but should not be constrained by theheadings set forth herein.

Use of broader terms such as “comprises,” “includes,” and “having”should be understood to provide support for narrower terms such as“consisting of,” “consisting essentially of,” and “comprisedsubstantially of.” Use of the terms “optionally,” “may,” “might,”“possibly,” and the like with respect to any element of an embodimentmeans that the element is not required, or alternatively, the element isrequired, both alternatives being within the scope of the embodiment(s).Also, references to examples are merely provided for illustrativepurposes, and are not intended to be exclusive.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods may beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another system,or certain features may be omitted or not implemented.

Also, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as directly coupled or communicating witheach other may be indirectly coupled or communicating through someinterface, device, or intermediate component, whether electrically,mechanically, or otherwise. Other examples of changes, substitutions,and alterations are ascertainable by one skilled in the art and could bemade without departing from the spirit and scope disclosed herein.

What is claimed is:
 1. A gas detector comprising: one or more sensingelements operable to detect volatile organic compounds (VOC); and aprobe operable to provide air flow into the gas detector, wherein theprobe comprises: a housing operable to attach to a main body of the gasdetector, and operable to hold a calibration tube during calibration ofthe gas detector; a cap operable to attach to the housing, and operableto fit over and cover the calibration tube; and one or more inlets,wherein when the calibration tube is inserted into the probe, the airflow is through the calibration tube, wherein the calibration tubeinserted into the probe comprises activated carbon fiber (ACF) filtermaterial, wherein the ACF filter material is pretreated by a methodcomprising: heating the ACF filter material to 160° C. for approximately1.5 hours; enclosing the ACF filter material in a glass chamber with asalt bath solution; and allowing the ACF filter material and the saltbath solution to equilibrate for at least 24 hours.
 2. The gas detectorof claim 1, wherein the calibration tube inserted into the probecomprises the ACF filter material which has been pretreated with asodium bromide (NaBr) solution.
 3. The gas detector of claim 1, whereinthe calibration tube inserted into the probe comprises broken glassends, and wherein the broken glass ends allow an air flow into and outof the calibration tube.
 4. The gas detector of claim 1, wherein theprobe further comprises threads between the housing and the cap.
 5. Thegas detector of claim 1, wherein the probe further comprises grippingelements on an exterior of at least one of the housing and the cap. 6.The gas detector of claim 1, wherein the probe further comprises anO-ring operable to seal with one end of the calibration tube when it isinserted into the housing.
 7. The gas detector of claim 1, wherein theprobe further comprises air inlets on the housing.
 8. The gas detectorof claim 1, wherein the probe further comprises air inlets on the cap.9. A calibration device comprising: a housing operable to attach to amain body of a gas detector, and operable to hold a calibration tubeduring calibration; a cap operable to attach to the housing, andoperable to fit over and cover the calibration tube; and one or moreinlets, wherein when the calibration tube is inserted into thecalibration device, an air flows through the calibration tube, andwherein the calibration tube comprises activated carbon fiber (ACF)filter material pretreated to enable the ACF filter material to maintaina humidity level greater than zero, wherein the ACF filter material isretreated by a method comprising: heating the ACF filter material to160° C. for approximately 1.5 hours; enclosing the ACF filter materialin a glass chamber with a salt bath solution; and allowing the ACFfilter material and the salt bath solution to equilibrate for at least24 hours.
 10. The calibration device of claim 9, wherein the gasdetector is operable to detect volatile organic compounds (VOC).
 11. Thecalibration device of claim 9, wherein the salt bath solution comprisesan alkali metal halide solution, potassium carbonate, magnesium nitrate,or potassium sulfate.
 12. The calibration device of claim 9, wherein thecalibration tube further comprises a first tip on one end of thecalibration tube and a second tip on another end of the calibrationtube, wherein the first tip and the second tip are broken during thecalibration of the gas detector.
 13. A method for manufacturing acalibration tube for use with a gas detector comprising: providing anactivated carbon fiber (ACF) filter material; pretreating the ACF filtermaterial to enable the ACF filter material to maintain a humidity levelgreater than zero, wherein pretreating the ACF filter materialcomprises: heating the ACF filter material to 160° C. for approximately1.5 hours; enclosing the ACF filter material in a glass chamber with asalt bath solution; and allowing the ACF filter material and the saltbath solution to equilibrate for at least 24 hours; placing the ACFfilter material into the calibration tube; and sealing the calibrationtube.
 14. The method of claim 13, wherein the salt bath solutioncomprises an alkali metal halide solution, potassium carbonate,magnesium nitrate, or potassium sulfate.
 15. The method of claim 13,further comprising: breaking a portion of the calibration tube to allowair flow through the calibration tube; inserting the calibration tubeinto a first portion of a probe attached to the gas detector; attachinga second portion of the probe onto the first portion, wherein the secondportion covers the calibration tube; and completing calibration of thegas detector while the calibration tube is inserted into the probe. 16.The method of claim 14, wherein the gas detector is operable to detectvolatile organic compounds (VOC).
 17. A calibration tube for use incalibrating a gas detector, the calibration tube comprising: acylindrical glass casing defining an interior space; an activated carbonfiber (ACF) filter material disposed within the interior space of thecylindrical glass casing, wherein the ACF filter material is pretreatedto enable the ACF filter material to maintain a humidity level greaterthan zero by a method comprising: heating the ACF filter material to160° C. for approximately 1.5 hours; enclosing the ACF filter materialin a glass chamber with a salt bath solution; and allowing the ACFfilter material and the salt bath solution to equilibrate for at least24 hours; a first breakable end configured to seal a first end of thecylindrical glass casing; and a second breakable end configured to seala second end of the cylindrical glass casing.
 18. The calibration tubeof claim 17, wherein the salt bath solution comprises an alkali metalhalide solution, potassium carbonate, magnesium nitrate, or potassiumsulfate.
 19. The calibration tube of claim 17, wherein the calibrationtube is configured to be received into a calibration device, wherein thecalibration device comprises: housing configured to attach to a mainbody of the gas detector and configured to hold the calibration tubeduring calibration; a cap configured to attach to the housing so as tofit over and cover the calibration tube; and one or more inletsconfigured to allow air-flow via the calibration tube when thecalibration tube is received into the calibration device.